1. Field of the Invention
This invention relates to a cutting feed starting position control device for numerically controlled machine tools such as grinding machines, milling machines and lathes, which is adapted to shift a starting position of a tool such as a grinding wheel or milling cutter responsively to a variation in size of the tool due to a regrinding of the tool as dressing or truing, and return the moved starting position to an original position when it is desired such as a change of grinding wheels.
2. Description of the Prior Art
Numerically controlled grinding machines have been widely used in machining shops. In the prior art numerical grinding machine control, an arithmetic unit is necessary for compensating a depth of cut due to a reduction in diameter of a grinding wheel whenever it is dressed or trued. Such a compensating unit however is complicated and high expensive. Moreover, the reduction in diameter of the tool each time of dressing or truing results in an extension of a cycle time of the operation. If an electric current is stopped in working operation of a machine for an emergency or unintentional reason, numerical control means is instantaneously brought into a reset condition which makes it difficult to restore the means to its original condition. In addition, when the numerical control means is supplied with electric current, a pulse motor in the apparatus rotates, at the most, pulses corresponding to one half of its pole number in a normal or reverse direction without receiving any information owing to an excitation phase shift phenomenon which occurs in a numerical control apparatus, and the undue rotation of the pulse motor results in a cumulative error which affects the operation of the machine till the last. In the prior art numerical control, a returning of moving parts to an original position is carried out by reading a preset value which has been preset for quick and cutting feeds, so that there is a problem how to remove a back-lash of the moving parts because the direction of the returning is opposite to that of the preceding feed. Furthermore any error in a preset feed or in a feed in an opposite direction by reading will become a cumulative error.